Example of how to choose a credit spread:

GOOG was trading at $516 on 16 July, with a strong downtrend.  A 580/590 call credit spread 31 days out cost $1.75, and the volatility was 46.44. Enter the Current Price ($516), the Days Ahead (31) and the Future Volatility (46.44%).  Enter 580 as the first target and 590 as the second, and click on GO!   The calculator comes back with the statement that the “Probability of Finishing below the lowest target (i.e. $580) is 80%”.   This is then a safe option to sell.   One option costs $175, which is 17.5% ROM.   Pretty good return!